Sputtering

ABSTRACT

This disclosure shows means to achieve controlled coatings by sputtering. By incorporating in a conventional sputtering system means to form a controllable sheet of plasma and means to cause a predetermined, continuous relative movement between the plasma and the substrate, control of deposition of the target material is achieved. This technique may be used to achieve a uniformity over wider surface areas and may also be used to achieve controlled non-uniformity of deposited coatings.

April 1972 D. HAJZAK 3,654,123

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United States Patent O 3,654,123 SPUTTERING Dennis G. Hajzak, Rochester, N.Y., assignor to The Bendix Corporation Filed Sept. 25, 1968, Ser. No. 762,436 Int. Cl. C23c 15/00 US. Cl. 204298 5 Claims ABSTRACT OF THE DISCLOSURE This disclosure shows means to achieve controlled coatings by sputtering. By incorporating in a conventional sputtering system means to form a controllable sheet of plasma and means to cause a predetermined, continuous relative movement between the plasma and the substrate, control of deposition of the target material is achieved. This technique may be used to achieve uniformity over wider surface areas and may also be used to achieve controlled non-uniformity of deposited coatings.

SUMMARY OF THE INVENTION The present invention relates to the art of sputtering. Sputtering is the process of removing molecular or atomic sized particles from a target of a selected material by controlled ion bombardment and depositing these particles to form a coating of the selected material on a selected substrate. It was the early practice to form the target as the cathode and to place the substrate near the anode in an inert, low-pressure atmosphere. This method, known as diode sputtering, was soon supplanted with a variety of more efficient methods.

One of these more efiicient methods involved forming the target and substrate separately from the anode and cathode and to locate them on opposite sides of a line drawn between the anode and cathode. Briefly, this system, known as triode sputtering, operated as follows: When placed in a low-pressure, inert atmosphere and with the anode held positive with respect to the cathode, electrons emitted by the cathode would be attracted toward the anode. The release of electrons could be accomplished by placing a high potential between the anode and cathode or by using a heated or thermionic cathode. The latter is most common. The electron current between cathode and anode causes a gas discharge, or plasma, to form in the space between the electrodes, due to ionization of the inert gas. This plasma consists of a space charge of negative electrons and a similar space charge or cloud, or positive ions leaving the plasma itself essentially neutral.

The target and substrate are placed on opposite sides of the plasma, intermediate the electrodes, and the target is maintained at a potential negative with respect to the ionized plasma. This serves to attract the positively charged plasma ions toward the target. Many of these ions arrive at the target surface with a level of energy and angle of incidence sufficient to dislodge molecular or atomic-sized particles of the target material which rebound across the plasma to the substrate where they form a deposited coating. Because the ions may strike the target from a variety of directions, interior surfaces of the vacuum system other than the substrate, are also coated. To achieve uniformity, the target is usually much larger than the substrate. Because of the ditfuseness of the plasma and the randomness of the ion bombardment of the target and subsequent target material migration, the rate of deposition of target material is low. In order to improve the deposition rate, the ion bombardment is concentrated by shaping and condensing the plasma through the use of a properly-oriented magnet or electro-magnet structure.

The use of thermionic cathodes leads to the presence Patented Apr. 4, 1972 of cathode material, as a contaminant, in the sputtered coating and on the target. This is true, though to a lesser degree, with non-thermionic cathodes as well. This contamination occurs because some positive ions from the plasma will be attracted to the cathode where they can cause particles of cathode material to be released by the cathode, that is, cathode sputtering will occur. Furthermore, the temperatures attained with the thermionic cathodes are often sufficient to evaporate cathode material. Since release of cathode material is virtually inevitable with both types of cathodes, regardless of the type which is used, the cathode is almost always placed within a conduit-like structure having at least one bend or curve. The conduit communicates directly with the interior of the vacuum system, but the cathode is so placed within the conduit that it cannot be seen from the vacuum system opening of the conduit. This placement requires that electrons emitted by the cathode cannot travel directly toward the anode, but rather, they must execute at least one change of direction in order to travel from the cathode to the conduit opening at the vacuum system.

This combination of circumstances presents the problem which it is the principle objective of this invention to solve. The electromagnetic field used to condense and shape the plasma is oriented generally parallel to the line of electron flight from the conduit opening to the anode. However, in order for the electrons to arrive at the conduit opening, they must have traveled within the conduit in a direction which was not parallel to the electromagnetic field. It is known that electrons traveling through a magnetic (or electromagnetic) field experience a force whose magnitude depends on the electric charge of the electron, magnetic (or electromagnetic) field strength, direction of motion and electron velocity perpendicular to the field. This force is then exerted on the electrons in a direction generally perpendicular to the field. Since the electrons are flowing within a confining structure, this has the effect of causing the electron density to increase greatly on one side of the conduit and to become virtually zero on the other side of the conduit. A plasma generated by this type of electron flow tends to exhibit the same relative densities as exhibited by the electron flow. It is, therefore, an object of this invention to provide a means for overcoming this electron shift and concomitant gradient in plasma intensity. In light of the foregoing objective, it is a further objective of this invention to provide means for utilizing the electron shift to produce a time-averaged plasma uniformity as well as to provide means which may be used to prevent the electron shift from occurring. It is also an object of this invention to provide a means of achieving wide area uniform coatings by sputtered deposition. Furthermore, it is an object of this invention to achieve this wide-area uniforinity while maintaining high deposition rates. Since sputtering is often used to deposit successive layers of material on a single substrate, it is a still further object of the present invention to provide a sputtering operation which provides for wide-area uniformity of deposited coating in which the substrates are maintained in a horizontal position for easy, safe, conveyor-belt transportation to a subsequent sputtering location.

Besides the two sputtering techniques discussed above, there are other techniques known to the skilled artisan such as A-C sputtering and asymmetric A-C sputtering. All of the known sputtering techniques are faced with the problems of uniformity of coating and the limited surface area over which the deposited coating will maintain the desired uniformity. It, therefore, becomes a still further object of the present invention to provide a means, applicable to any sputtering apparatus, to achieve wide-area uniformity of sputtered coatings.

Recognizing that it is known to the man skilled in the art that the substrate and/or target may be moved or located in a predetermined manner in order to achieve a desired deposited coating, it is also an object of this invention to provide a means for sputter coating a desired material in a desired manner without having to vary the physical relationship between the target and the substrate. Imparting mechanical motion to the substrate or varying the relative positions of the target and substrate is an often tedious task requiring that the vacuum system be brought up to atmospheric pressure to allow an operator to physically move substrate and target. Alterna tively, mechanical motion may be transmitted into the existing vacuum environment through the proper types of feedthrough mechanisms, but these are expensive and leave accuracy of positioning to depend upon the skill of the operator. It is, therefore, a still further object of this invention to provide means, controllable externally of the vacuum to controllably vary the sputtered coating, which means do not require variation in the relative positions of the target and substrate.

The present invention is concerned with providing means, magnetic and electrical, for providing a periodic, cyclic variation between the relative positions of the plasma, the substrate, and the target or for providing a wide plasma of uniform density.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a preferred embodiment of the present invention as it would be applied to a sputtering system contained within an evacuated bell jar.

FIG. 2 shows the sputtering system of FIG. 1 using magnetic means to achieve the objects of this invention.

FIG. 3 shows a sputtering system using electrical means to achieve the objects of this inventon.

DETAILED DESCRIPTION OF THE DRAWING Referring now to FIG. 1 of the drawing, a preferred embodiment of my invention is shown in conjunction with a typical vacuum system, indicated generally by 10. The vacuum system consists of a bell jar 12 in vacuum-tight relationship with a base 14 through feedthrough ring 16. The interior of the bell jar 12 is in communication, through conduit 19 with a pumping system, not shown. The pumping system may be any of the known systems and, indeed, the entire vacuum system, so far discussed, is well-known to the practitioner of the art and need not be further discussed.

In a sputtering operation, the interior region of the vacuum system 10 is first pumped down to achieve a very low absolute pressure (high vacuum). The interior is then back-filled from. means not shown through conduit 11 controlled by valve 13 with a small volume of a selected inert, ionizable gas. The plasma necessary for sputtering is then formed by ionization of the selected gas. In order to ionize the gas, a cathode 20 and anode 22 are located within the bell jar 12, juxtaposed to form the plasma therebetween. In this instance, a thermionic cathode is shown, but it could also be a cold cathode. Cathode 20' is connected by electrical leads 15, which pass through feedthrough ring 16 by way of conduit 18, to a power supply, not shown. As hereinabove discussed, the cathode 20 is hidden within an electron deflector 24 in order to prevent the possibility that cathode material may emerge from the electron deflector to contaminate the system. The anode 22 is horizontally to the side of and above the cathode 20 so as to be directly across from the opening in the electron deflector 24 as best shown in FIG. 2. In this embodiment, the anode 22 is supported by electrically-insulating support means 26. The anode 22 is electrically-biased to a value more positive than the cathode 20 so the electrons emitted by the cathode 20 will tend to flow towards the anode 22. The electrical circuit for this is well-known and is not shown so as to improve clarity and because it forms no portion of the area of invention. A target 28 is positioned to be in the region of the plasma and slighty above it, while a substrate 30, shown by phantom lines, lies below the target 28. The target and substrate are shown unsupported only for illustrative purposes since in the practice of this invention it would, of course, be necessary to support the target and substrate in the indicated positions.

To minimize electrical effects and contamination from the pumping system, feedthrough ring 16 and base 14, an isolated shield plate 32 may be placed between the electrodes 20 and 22 and base 14 and positioned by insulator, two of which, 34 and 36, are shown. A gap for pumping gas is maintained between the periphery of shield plate 32 and bell jar 12.

Externally of the bell jar 12 is an electromagnetic flux means 38 consisting of electromagnetic coils 40 and 42, flux strap 44, curved flux distributing plates 46 and 48, and signal generating means 50. The signal generating means 50 are shown connected to only one coil, however, the second coil may be electrically-intercomnected to the first or to its own signal generating means or to the first signal generating means 50. Alternatively, only one coil could be used in generating the required electromagnetic flux. Flux distributing means 46 and 48 are operative to generate an essentially uniform flux or B- field within the bell jar 12 which will serve to condense and confine the plasma to the region of the target material 28 with flux strap 44 operative to confine the flux outside of the bell jar 12. This helps to prevent stray flux from aflecting the signal generating means and any other equipment in the region of the bell jar 12. The illustrated flux means 38 represents only one of the known means for producing uniform flux or B-fields. The signal generating means 50 is operative to produce a variable voltage and/or alternating polarity signal to periodically cyclically vary the excitation of coil 40' to vary the strength and orientation of the generated B-field. The importance of this variation will be explained hereunder.

Referring now to FIG. 2, the essential components of the preferred embodiment of our invention are shown, but with the target 28 and substrate 30 removed for clarity. The flux producing means 38 are not shown, but are represented by the B and arrow which indicate one direction of the flux lines of the B-field. As can be seen, this is essentially parallel to the line drawn between the center of the opening of the electron deflector 24 and the center of the anode 22. Because the electrons emitted by the cathode 20 must move upwardly before they can exit from the electron deflector 24, they will cross electromagnetic flux lines. This crossing imparts to these electrons a force operative to cause an accumulation of electrons to one side of the deflector 24. This results in nonuniformity in the plasma density due to the non-uniform distribution of electrons. Accordingly, my invention provides means for correcting this non-uniformity of electron distribution. From a consideration of FIGS. 1 and 2, it will be observed that the coil exciting or signal generating means 50 will produce a cyclic variation in field strength and in orientation which Will be operative to cause the region of electron density to alternately switch from one side of the electron deflector 24 to the other. Depending on the eifect desired, this will operate to produce a time-averaged uniformly dense electron emission from the deflector 24 or can cause the time-averaged density to vary in accord with a predetermined sputting rate. The opening in the electron deflector 24 is shaped as an elongated rectangle in order to achieve a sheet-like emitted electron flow and plasma, but without the shaping provided by the B-field, the electrons would tend to repel one another and form a cone-like plasma. Provision of a periodically-cycling B-field overcomes the aforementioned difficulty caused by using a B-field to control the plasma shape.

FIG. 3 shows another form of my invention as applied in a somewhat different electron deflector 124. The electron deflector is shown with a portion removed so that the interior of the deflector 124 is exposed. This deflector differs from the deflector shown in FIGS. 1 and 2 in that the cathode 120 is located closer to the base of the deflector. Placement of the cathode may be at any location within the electron deflector 124 provided the location does not provide a straight line path to the month 125 of the electron deflector 124. In some instances where thermionic cathodes are used, it is the practice to locate the cathode at the feedthrough rings to permit easy access to replace the cathode when it burns out. Located within the deflector 124 are a pair of electrostatic deflection plates 160 and 162 which are shown as electrically-connected to signal generator 150 by appropriate leads. The generator would ordinarily be located externally of the vacuum environment to permit an operator to vary the electrostatic charge on plates 160 and 162. These plates 160 and 162 would be charged so that electrons passing between them would experience a constant electrostatic force urging them in opposition to the force exerted on them by the B-field. This would have the elfect of causing the electrons to maintain a uniform density during that portion of their flow when they would ordinarily tend toward one side of the electron deflector 124. Alternatively, in the absence of a B-field, these electrostatic deflection plates 160 and 162 could be used to cause the electron flow to be more dense in one side of the deflector or the other in order to produce a plasma having a predetermined time-averaged non-uniformity so that the sputtering rate on a substrate could result in a selected non-uniformity. Plates 160 and 162 could also be used in the absence of a B-field to produce a time-averaged uniformly dense plasma.

It can be seen that the present invention accomplishes its stated objectives in providing means to achieve controlled sputtered coating by providing means periodically varying the emitted electron density to achieve timeaveraged uniform electron density. Further, my invention may be used to provide either a selectively variable, predetermined non-uniformity in emitted electron density or a wide area uniformity in emitted electron density.

While reference has been made hereinabove to lowpressure plasmas of inert gas, it is known that reactive gases such as oxygen and nitrogen may also be constituents of the plasma and the scope of the invention is not intended to be limited by the above-noted references to inert gas plasmas.

I claim:

1. A triode sputtering apparatus for depositing a thin film of a material onto a substrate from a target of said material, which comprises:

an enclosure;

means for evacuating said enclosure and for providing an ionizable atmosphere in the enclosure;

means for mounting said substrate in said enclosure;

means for mounting said target in said enclosure in spaced relationship to said substrate;

an anode disposed in said enclosure;

an electron deflector having an opening therein disposed in said enclosure, said opening facing said anode and the space between said target and substrate;

a cathode disposed in said electron deflector and operable to provide an electron discharge between said anode and cathode and establish an ion plasma between said target and said substrate;

electromagnetic flux means for establishing in said enclosure a magnetic field having longitudinal field lines substantially parallel to the surface of said substrate to be coated and said ion target to be sputtered; and

means for generating an AC signal in circuit relationship with said electromagnetic flux means to produce a variable voltage which continuously and periodically varies the magnitude and direction of said magnetic field lines.

2. The combination as recited in claim 1 wherein said cathode is disposed Within said deflector so that it does notprovide a straight line path between said cathode and said opening of said deflector whereby material sputtered by said cathode does not exit said deflector and strike said substrate.

3. The combination as recited in claim 2 wherein the opening in said electron deflector is rectangularly shaped.

4. The combination as recited in claim 2 wherein said cathode is a thermionic cathode.

5. The combination as recited in claim 1 wherein said magnetic field generating means includes flux distributing means external said enclosure to confine said flux.

References Cited UNITED STATES PATENTS 3,021,271 2/1962 Wehner 204l92 3,393,142 7/1968 Moseson 204-298 3,410,775 11/1968 Vratny 204-192 JOHN H. MACK, Primary Examiner S. S. KANTER, Assistant Examiner 

